Polymeric foams can be generally classified as either closed-cell foams or as open-cell foams. Open-cell foams can be used as a matrix to contain various liquids and gases. They are suitable for various applications such as, for example, use in wipes and diapers, as carriers and ion exchange resins. For some of these applications, it is desirable to have porous crosslinked polymer blocks which have a very low density and a high capacity of absorbing and retaining liquids. Such high absorption capacity, low density, porous polymer blocks can be prepared by polymerizing a specific type of water-in-oil emulsion known as high internal phase emulsion having relatively small amounts of a continuous oil phase and relatively greater amounts of an internal water phase.
Such high absorption capacity, low density, foams are prepared by a process disclosed in U.S. Pat. No. 4,522,953 by polymerizing and crosslinking monomers in the continuous oil phase of a high internal phase water-in-oil emulsion with a polymerization initiator such as potassium persulfate. Generally, these high internal phase water-in-oil emulsions contain at least 90 weight percent of a water phase as the internal phase. The high ratio water-in-oil emulsions are formed by combining the oil phase with water under moderate shear. In order to obtain this high internal phase water-in-oil emulsion, a surfactant must be used to stabilize the emulsion. It is also advantageous to incorporate an electrolyte into the aqueous phase. The amount and type of electrolyte, along with the amount and type of surfactant, effects the pore size, and wicking ability of the cured foam.
Certain properties of foams can be improved by selection of monomers. For example, strength of the cured foam can be significantly improved by incorporating styrene as a monomer, and softness is generally undesirably impacted by increased levels of styrene monomer. Monomers such as 2-ethylhexyl acrylate provide good absorbency, but result in a weak foam. The total amount of liquid that a foam can hold under load is a function of both absorbency, and the strength of the foam, because weight of adsorbed liquid will cause a weak foam to collapse, thus squeezing out adsorbed liquid. Foams made from high internal phase emulsions have therefore typically incorporated a mixture of monomers to obtain a compromise of properties.
Pore size and density of the foams are also dictated by compromises between properties of the foam product.
It is also desirable for some applications of high internal phase emulsion foams to have both hydrophilic and hydrophobic regions within the same foam. Such properties would be useful, for example, as a medium for removing hydrocarbons from a stream of water. U.S. Pat. No. 5,061,767, issued on Oct. 29, 1991, discloses a polymer composite useful as permselective membranes by polymerizing an emulsion of hydrophobic and hydrophilic monomers. Preparation of a foam of having both hydrophobic and hydrophilic regions is not suggested.
Composites of foams are disclosed in U.S. Pat. No. 5,037,859. These foams are rigid foams and are prepared by first curing an emulsion to form a rigid open cell foam, such as a polystyrene foam, having a relatively large pore size, and then forcing a second emulsion into the cured polystyrene to form a smaller pore diameter foam within the polystyrene. The resultant composite foams have greater strengths and densities then foams from the first emulsion, and retain the better wicking properties of the first foam. Polymerizing an emulsion within another foam matrix inherently increases density of the resultant foam composite and therefore decreases absorbency. It would be preferred to provide a composite foam wherein properties such as absorbency are not significantly compromised in order to achieve improvements in strength and other properties.
It is therefore an object of the present invention to provide a method to prepare foams from mixtures of high internal phase emulsions wherein the resultant foams exhibit characteristics of each of two separate foam compositions, or a more desirable combination of the properties compared to a high internal emulsion foam of the components of the combined two emulsions.